1. Field of the Invention
This invention relates to solid electrolytic capacitors such as solid tantalum or aluminum capacitors. More specifically, the present invention relates to a solid electrolytic capacitor of the type which has a built-in fuse for improving the safety of the capacitor.
2. Description of the Prior Art
A solid electrolytic capacitor having a built-in safety fuse is disclosed for example in "NEC Technical Report" Vol. 44, No. 10/1991, Pages 116-120 or Japanese Patent Application Laid-open No. 2-105513. Such a capacitor is also illustrated in FIGS. 12-14 of the accompanying drawings.
As shown in FIGS. 12-14, the prior art solid electrolytic capacitor comprises a capacitor element 101 which includes a chip body 101a (sintered mass of metal particles) and an anode wire 101b projecting from the chip body. The capacitor further comprises an anode lead 102 electrically connected to the anode wire 101b by welding for example, and a cathode lead 103 electrically connected to the chip body 101a through a fuse wire 104. The fuse wire 104 may be made to break upon overheat in the case of a temperature fuse or upon passage of an overcurrent in the case of an overcurrent fuse.
The fuse wire 104 is partially enclosed in a relatively soft arc-extinguishing member 106 which may be made of silicone resin. The capacitor element 101 together with the fuse wire 104 and part of the respective leads 102, 103 is enclosed in a protective package 105 which is made of a relatively hard resin such as epoxy. The projecting portions of the respective leads 102, 103 are bent to engage the underside of the package 105.
In such a prior art capacitor, one end (first end) 104a of the fuse wire 104 is connected to the cathode lead 103 by causing a bonding tool 107 to press the first end 104a, as best shown in FIG. 14. As a result, the first end 104a is flattened for bonding to the cathode lead 103 with a sufficient adhesion area.
However, the above manner of bonding is disadvantageous in the following respects.
(1) Since the first end 104a of the fuse wire 104 is cross-sectionally reduced due to flattening, the fuse wire 104 tends to become extremely weak near the flattened first end 104a. Thus, at the time of molding the resin package 105, the fuse wire 104 is likely to break near the flattened first end 104a under the influences of the resin injection pressure.
(2) The breaking temperature and/or current of the fuse wire 104 are determined by the cross-sectionally smallest portion of the fuse wire which is located near the flattened first end 104a. Since the degree of flattening cannot be strictly equalized with respect to different fuse wires, it is inevitable that the breaking characteristics of different products varies.
(3) The flattened first end 104a of the fuse wire 104 must have a sufficient length H (see FIG. 12) for providing a sufficient bonding strength. Thus, the bonding portion of the cathode lead 103 must be correspondingly elongated to result in an increase of the length L1 of the capacitor, thereby hindering a reduction in the size and weight of the product.
On the other hand, it is also possible to connect the first end 104a of the fuse wire 104 to the cathode lead 103 by soldering without inviting the problems (1) and (2) described above. However, the soldering operation is relatively time-taking, and the necessity of separately using solder adds to the production cost. Further, the problem (3) above cannot be fully solved by the use of solder.